Open Access
Add to BibSonomy
Abstract
In this paper, we report on the experimental observation of harmonic mode locking (HML) fiber laser based on evanescent field of tapered fiber using topological insulator (TI) Bi2Te3 saturable absorber (SA). The optical deposition method was adopted to fabricate the tapered fiber-based TISA. By significant nonlinear optical property of the tapered fiber TISA and high nonlinear fiber (HNLF), the fiber laser could operate at the pulse repetition of 3.125 GHz under 976 nm pump power of 148 mW, the pulse width of 1.754 ps, and the output power of 6.4 mW. Our results demonstrate that the tapered fiber TI device and HNLF can act as both high nonlinear optical component and SA in fiber lasers, and could also have better performance to achieve ultrashort pulses with ultrahigh repetition.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Passively mode-locked fiber laser had attracted much attention because of the high energy pulse generation and widely used in many applications, such as laser imaging, military, medicine and material processing. To achieve passive mode locking in fiber laser, the key element is the saturable absorbers (SAs). To date, several approaches have been proposed to achieve the saturable absorption in a mode-locked fiber laser, such as semiconductor saturable absorption mirror (SESAM), single wall carbon nanotube (SWCNT) and graphene [1–7]. However, the drawbacks of SESAM are the high cost and limited range of response bandwidth. The modulation depth of carbon based materials is low, which limits the pulse energy.
In recent years, topological insulators (TIs) have been considered as efficient SA for mode-locked fiber laser due to its similar optical characters and electronic band structure to graphene [8–10]. Moreover, TIs have exhibited the ultra large saturable intensity, and have a large nonlinear refractive index as the increase pump power. Therefore, TIs have been the object of extensive experimental research in recent years [11–16]. In 2014, J. Sotor et al. experimental demonstrated the ultrafast fiber laser mode locking by inserting the bulk Sb2Te3-based SA into the laser cavity [17]. In addition, Liu et al. proposed a hybrid mode-locked method based on Sb2Te3 and nonlinear polarization evolution (NPE), and 70 fs pulses at 1542 nm were obtained [18]. However, when the pump power is further increased, the high nonlinear effect is introduced into the laser cavity, and the passive mode-locked fiber laser will generate two related states: pulse beam and harmonic mode locking (HML) [19–25]. The pulse beam is mainly due to the strong nonlinear effect that makes the pulse energy quantized, and resulting in pulse splitting. The generation of HML was generated by adjusting the polarization and nonlinearity of the cavity, so that each pulse beam has the same time interval. The repetition of high-order HML can be multiple increased up to the order of GHz. In 2014, Wang et al. demonstrated a maximum of 17 pulses in a soliton beam by NPR, and the 15 order harmonic mode was realized [26]. 2016, Jia et al. reported that the maximum 128 order harmonic mode locking was achieved by NPR in thulium doped fiber laser, but the intensity jitter of the pulse was larger [27]. In previous studies, most fiber laser realized HML by NPR.
In this paper, we will address the issue of pulse jitter of HML. The passive HML in the fiber laser by a microfiber-based TISA was demonstrated, and the high nonlinear fiber (HNLF) was introduced in the cavity. With the proper adjustment of cavity parameters, 3.125 GHz repetition rate pulse was achieved, which corresponds to the 200 th harmonic of fundamental cavity frequency. The obtained results indicate that HNLF and evanescent field can replace single mode fiber to strengthen the nonlinearity and shorten the cavity length, and it is conducive to the stability of fiber laser. Moreover, by adjusting the polarization state, the filtering effect was formed in the cavity by the interacting between the polarization and birefringence of HNLF, which reduced the threshold of harmonic mode locking.
2. Bi2Te3 preparation and experimental setup
The TI: Bi2Te3 nano-sheets were synthesized by hydrothermal method as previous reports [28]. The saturable absorber Bi2Te3 was prepared through the following process. PVP, NaOH, Na2TeO3 and BiCl3 were added to ethylene glycol in order and transparent solution was obtained by stirring sufficiently. The concentration of Bi and Te ions in the solution was 0.1 M. Then the solution was transferred into an autoclave with Teflon lining up to 80% of the capacity. The autoclave was heated at 190°C for 30 h. The powders were obtained by centrifugation, cleaning with distilled water and drying at 60°C. Figure 1(a) shows the X-Ray diffractometer (XRD) images and scanning electron microscope (SEM) of Bi2Te3 nano-sheets. We could obviously see that the Bi2Te3 nano-material has a two-dimensional sheet structure, which is a very thin layer with about 200 nm in width. The absorption of light by Bi2Te3 is shown in Fig. 1 (b). It can be seen that Bi2Te3 absorbs light in a very wide spectral range, indicating that it can be used as a saturated absorber in a wide spectral range.
Fig. 1 (a) XRD patterns of the as-prepared Bi2Te3 nano-sheets, inset: SEM imaging of prepared TI Bi2Te3 nano-sheets; (b) the linear optical absorption of the Bi2Te3.
The SA was fabricated by depositing the Bi2Te3 on the tapered fiber. The tapered fiber was fabricated by fiber tapering equipment (FSM-100P + ) with the SMF-28 fiber. The length and waist of the tapered fiber were about 2.5 mm and 18 μm respectively. The Bi2Te3 was deposited on the tapered fiber by pulsed laser deposition method. The specific process was that: the tapered fiber was immersed in Bi2Te3 solution, and a continuous wave laser source with an average power of 1 W passed through the tapered fiber for 60 minutes. The microscope image of the prepared SA is shown in Fig. 2(a). It can be seen that Bi2Te3 was uniformly coated on the surface of tapered fiber. The optical field distribution is shown in Fig. 2(b). Without the deposition, the laser scattering occurred at the both ends of the tapered fiber induced by encapsulation. After the deposition, obviously, laser was heavily scattered at the central part of the tapered fiber, which was caused by the leakage of the evanescent field due to the existence of high refractive index materials (TI: Bi2Te3 nano-sheets).
Fig. 2 (a) The optical field distribution of tapered fiber without Bi2Te3; (b) The optical field distribution of tapered fiber with Bi2Te3; (c) The microscope image of the tapered fiber TI: Bi2Te3.
In order to further investigate the characteristic of the TI: Bi2Te3, the experimental setup is shown in Fig. 3(a). The nonlinear optics response of TI: Bi2Te3 were investigated by using an in-house made femtosecond pulse source (center wavelength, 1556.12 nm; repetition rate, 15 MHz; pulse duration, ~600 fs). To increase the pulse energy, the output femtosecond pulse from the fiber laser was amplified by an Er-doped fiber amplifier (EDFA), and an adjustable optical attenuator was used to control the output power of the source. The beam was divided into two beams by a 3 dB coupler. One of them was directly connected to the power meter as reference beam. The other beam passed through the tapered fiber loaded with TISA, and then entered to the power meter. The corresponding result was shown in Fig. 3(b), the transmission of TISA was increased by about 1.82% (-modulation depth) to the level of 31%, and the nonsaturable loss is ~68.6%. However, the modulation depth of Bi2Te3 is a little low, which could be improved by increasing the intersect length of Bi2Te3 and evanescent field of laser.
Fig. 3 (a) Experimental setup for nonlinear absorption measurement of tapered-fiber based TISA; (b) The measured transmission points and the corresponding fitting curve.
3. Experimental results and discussion
The laser schematic used in our experimental demonstration was shown in Fig. 4. The mode locked Er-doped fiber laser was carried out by a ring cavity. The gain medium was a piece of 0.8 meters heavily doped Erbium-doped fiber (Er80-8/125, Liekki) with ~-20 ps2/km group velocity dispersion (GVD) and a peak core absorption coefficient of 64 dB/m at 980 nm, and 30 meters HNLF was introduced in the ring cavity. An intra-cavity polarization independent isolator (PI-ISO) was used to force the unidirectional operation in the ring cavity. A polarization controller (PC) was used to adjust the birefringence and optimize the laser spectrum of intra-cavity. A 980/1550 wavelength division multiplexer (WDM) was used to couple the pump laser into the ring cavity, the laser was pumped by a 980 nm single-mode semiconductor laser with ~400 mW (LC9XU400-74P, II-VI). A 10% output port from 90:10 fiber coupler was used to extract the laser from the ring cavity, the TISA covered on the tapered fiber. The total of the fiber laser ring cavity was about 45 m. The laser emitting was monitored simultaneously by an optical spectrum (AQ-6370D), oscilloscope (DSOV084A, Agilent) with a 12.5 GHz photodiode detector, and a commercial autocorrelator (Femtochrome, FR-103XL).
In the experiment, continuous wave operation started at a pump power at 15 mW and the self-started mode-locking occurred at about 28 mW. For stable performance of fiber laser, the mode-locked operation with the fundamental repetition rate of 15.6 MHz can be observed by carefully fine-adjusting the polarization controller at the pump power of 36 mW. Figure. 5 shows the typical mode-locked operation with the fundamental repetition rate. As can be seen in Fig. 5(a), the mode-locked spectrum center is 1560.88 nm, and the 3dB spectral bandwidth is 2.12 nm. Due to the HNLF and tapered fiber-based TISA, the proposed fiber laser always tends to operate in multipulse state by further improve the pump power. Then there were some continuous wave component coexists with the mode-locked pulse beam by reduce the pump power, which can be shown in the spectrum. As can be seen from Fig. 5(b), the full width at half maximum (FWHM) of the pulse is 2.18 ps, the pulse repetition rate is 15.6 MHz, and the pulse train in the range is 300 ns in the inset of Fig. 5(b).
Fig. 5 Mode-locked operation at fundamental repetition rate. (a) Mode-locked spectrum; (b) measured autocorrelation pulse trace, inset: corresponding oscilloscope trace of the output pulse train.
With the increase of the pump power, the PC within the cavity was finely adjusted. The pulse splitting was observed due to the soliton energy quantization effect [29]. Then, a typical characteristic of passive HML in fiber laser cavity was observed, that the pulse repetition rate increased with the increasing pump power. The fiber laser achieved above 3 GHz repetition rate at the pump power of 148 mW, that corresponding to 200 th harmonic of fundamental repetition rate. Figure 6(a) shows the mode-locked spectrum of HML at the repetition rate of 3.125 GHz. The center wavelength was 1560.88 nm, and the 3 dB spectral bandwidth was 1.52 nm. Compared with the former, the cw component disappeared due to the pulse splitting generated by high-order harmonics can suppress the generation of cw. It is worth noting that the Kelly sidebands at the edge of the spectrum disappear, this is mainly due to the influence of polarization state on Kelly sidebands in the experiments. The pulse train was shown in Fig. 6(b), the time interval of the adjacent HML pulses was 0.32 ns, and corresponding repetition rate was 3.125 GHz, and the output HML pulse width was expected to be 1.754 ps, which was shown in Fig. 6(c). According to the output pulse, a hyperbolic secant pulse profile is assumed. Considering the 3 dB bandwidth of HML pulses, the estimated time-bandwidth product (TBP) was 0.328, which was slightly larger than that of transform-limited sech2 pulses (~0.315).
Fig. 6 (a) Output spectrum of the mode-locking EDFL at the pump power of 148 mW; (b) the oscilloscope trace of the 200th harmonic output pulse train; (c) Pulse waveform of the mode-locked EDFL with a pulse separation of 1.754 ps.
Figure 7 shows the measured pulse energy and the pulse width of the output pulses as a function of the harmonic orders. The pulse energy enlarged with the increase of harmonic order near the mode locking threshold. However, the pulse energy decreased from 4.5 pJ to 2.05 pJ as the harmonic order was enlarged from to 15 ~200, it was owing to the HML leads to splitting of the pulses, the increase of pump power was not enough to compensate for the decrease of the pulse energy due to the pulse splitting. In the experiment, noting that when the laser achieved the mode-locking, the pulse energy is small, increasing the pump power does not accumulate the chirp effect, and the pulse splitting was not occurred, and the pulse energy increased with the enlarged pump power. The output pulse width was observed to be less than 2.2 ps for all harmonic orders. The pulse width varies slightly depending on the harmonic order. Even when the harmonic order was increased, the output pulses maintained their temporal width between 1.75 ps and 2.18 ps.
In order to better indicate the capability of HML fiber laser, the evolution of the experimentally measured the output power and harmonic number with respect to the pump power as show in Fig. 8. It can be clearly seen that the harmonic number linearly scales with the pump power level. The measured repetition rate from 15.6 MHz to 3.125 GHz, and maximum repetition rate corresponds to the 200 th harmonic order. Moreover, the type of linear relation that is indicated between the pulse output power from 0.09 to 6.4 mW and the pump power was observed when the pump power was adjusted from 15 to 148 mW, the estimate of pump harmonic efficiency is 4.3%, as shown in Fig. 8. It should be also noted that the optical damage of TISA could be the major limitation to further increase in output power and harmonic number. However, the optical damage of tapered fiber TISA was not observed but Q-switched mode-locking occurred by further increase the pump power. This is mainly due to bleaching of TISA caused by excessive energy, so the nonlinear absorption of the proposed TISA could be the major limitation to further scaling in harmonic number.
4. Conclusion
In summary, we have experimentally demonstrated a harmonically mode-locked and all-fiberized laser that can produce picosecond pulses at a repetition rate of 3.125 GHz, whereby nano-sheets, Bi2Te3-deposited tapered fiber was used as a fiberized SA; furthermore, HNFL was add to stable harmonically mode-locked pulses with a temporal width of 1.754 ps could readily be generated from an erbium-doped-fiber cavity at various harmonic orders, and the time-bandwidth product was 0.328, which was slightly larger than that of transform-limited sech2 pulses (~0.315). The maximum harmonic order was 200 th and its corresponding repetition rate was measured as 3.125 GHz. The observed results provided the demonstration of applications of both high nonlinear and saturable absorption effects, showing that the proposed tapered fiber TISA and HNLF could find important applications in the fields of nonlinear and harmonic mode locking.
Funding
National Natural Science Foundation of China (Grant No. 61805023); Planning Project of Jilin Provincial Education Department (JJKH20181112KJ); Jilin Science and Technology Development Plan (Grant No. 20180519018JH); Innovation Fund of Changchun University of Science and Technology (XJJLG-2017-08).
References
1. H. Zhang, D. Y. Tang, L. M. Zhao, X. Wu, and H. Y. Tam, “Dissipative vector solitons in a dispersionmanaged cavity fiber laser with net positive cavity dispersion,” Opt. Express 17(2), 455–460 (2009). [CrossRef] [PubMed]
2. A. Martinez, K. Zhou, I. Bennion, and S. Yamashita, “Passive mode-locked lasing by injecting a carbon nanotube-solution in the core of an optical fiber,” Opt. Express 18(11), 11008–11014 (2010). [CrossRef] [PubMed]
3. D. Popa, Z. Sun, F. Torrisi, T. Hasan, F. Wang, and A. C. Ferrari, “Sub 200 fs pulse generation from a graphene mode-locked fiber laser,” Appl. Phys. Lett. 97(20), 111112 (2010). [CrossRef]
4. J. Wang, X. Liang, G. Hu, Z. Zheng, S. Lin, D. Ouyang, X. Wu, P. Yan, S. Ruan, Z. Sun, and T. Hasan, “152 fs nanotube-mode-locked thulium-doped all-fiber laser,” Sci. Rep. 6(1), 28885 (2016). [CrossRef] [PubMed]
5. J. Wang, Y. Yan, A. P. Zhang, B. Wu, Y. Shen, and H. Y. Tam, “Tunable scalar solitons from a polarization-maintaining mode-locked fiber laser using carbon nanotube and chirped fiber Bragg grating,” Opt. Express 24(20), 22387–22394 (2016). [CrossRef] [PubMed]
6. A. Martinez, K. Zhou, I. Bennion, and S. Yamashita, “Passive mode-locked lasing by injecting a carbon nanotube-solution in the core of an optical fiber,” Opt. Express 18(11), 11008–11014 (2010). [CrossRef] [PubMed]
7. G. Sobon, A. Duzynska, M. Świniarski, J. Judek, J. Sotor, and M. Zdrojek, “CNT-based saturable absorbers with scalable modulation depth for Thulium-doped fiber lasers operating at 1.9 μm,” Sci. Rep. 7(1), 45491 (2017). [CrossRef] [PubMed]
8. L. L. Miao, J. Yi, Q. K. Wang, D. Feng, H. R. He, S. B. Lu, C. J. Zhao, H. Zhang, and S. C. Wen, “Broadband third order nonlinear optical responses of bismuth telluride nanosheets,” Opt. Mater. Express 6(7), 2244–2251 (2016). [CrossRef]
9. C. Tan, Q. K. Wang, and X. Q. Fu, “Topological insulator Sb2Te3 as an optical media for the generation of ring-shaped beams,” Opt. Mater. Express 4(10), 2016–2025 (2014). [CrossRef]
10. S. Q. Chen, C. J. Zhao, Y. Li, H. H. Huang, S. B. Lu, H. Zhang, and S. C. Wen, “Broadband optical and microwave nonlinear response in topological insulator,” Opt. Mater. Express 4(4), 587–596 (2014). [CrossRef]
11. C. J. Zhao, H. Zhang, X. Qi, Y. Chen, Z. T. Wang, S. C. Wen, and D. Y. Tang, “Ultra-short pulse generation by a topological insulator based saturable absorber,” Appl. Phys. Lett. 96(21), 211106 (2012). [CrossRef]
12. Y.-H. Lin, C.-Y. Yang, S.-F. Lin, W.-H. Tseng, Q. Bao, C.-I. Wu, and G.-R. Lin, “Soliton compression of the erbium-doped fiber laser weakly started mode-locking by nanoscale p-type Bi2Te3 topological insulator particles,” Laser Phys. Lett. 11(5), 055107 (2014). [CrossRef]
13. H. Zhang, X. He, W. Lin, R. Wei, F. Zhang, X. Du, G. Dong, and J. Qiu, “Ultrafast saturable absorption in topological insulator Bi2SeTe2 nanosheets,” Opt. Express 23(10), 13376–13383 (2015). [CrossRef] [PubMed]
14. F. Bonaccorso and Z. P. Sun, “Solution processing of graphene, topological insulators and other 2d crystals for ultrafast photonics,” Opt. Express 4(1), 63–78 (2013). [CrossRef]
15. Y. Meng, G. Semaan, M. Salhi, A. Niang, K. Guesmi, Z. C. Luo, and F. Sanchez, “High power L-band mode-locked fiber laser based on topological insulator saturable absorber,” Opt. Express 23(18), 23053–23058 (2015). [CrossRef] [PubMed]
16. Q. Wang, Y. Chen, L. Miao, G. Jiang, S. Chen, J. Liu, X. Fu, C. Zhao, and H. Zhang, “Wide spectral and wavelength-tunable dissipative soliton fiber laser with topological insulator nano-sheets self-assembly films sandwiched by PMMA polymer,” Opt. Express 23(6), 7681–7693 (2015). [CrossRef] [PubMed]
17. J. Sotor, G. Sobon, K. Grodecki, and K. M. Abramski, “Mode-locked erbium-doped fiber laser based on evanescent field interaction with Sb2Te3 topological insulator,” Appl. Phys. Lett. 104(25), 251112 (2014). [CrossRef]
18. W. Liu, L. Pang, H. Han, W. Tian, H. Chen, M. Lei, P. Yan, and Z. Wei, “70-fs mode-locked erbium-doped fiber laser with topological insulator,” Sci. Rep. 6(1), 19997 (2016). [CrossRef] [PubMed]
19. Y. Meng, S. Zhang, X. Li, H. Li, J. Du, and Y. Hao, “Multiple-soliton dynamic patterns in a graphene mode-locked fiber laser,” Opt. Express 20(6), 6685–6692 (2012). [CrossRef] [PubMed]
20. Y. F. Song, L. Li, H. Zhang, D. Y. Shen, D. Y. Tang, and K. P. Loh, “Vector multi-soliton operation and interaction in a graphene mode-locked fiber laser,” Opt. Express 21(8), 10010–10018 (2013). [CrossRef] [PubMed]
21. M. S. Kang, N. Y. Joly, and P. S. Russell, “Passive mode-locking of fiber ring laser at the 337th harmonic using gigahertz acoustic core resonances,” Opt. Lett. 38(4), 561–563 (2013). [CrossRef] [PubMed]
22. T. H. Hu, G. B. Jiang, and Y. Chen, “Passive harmonic mode-locking in Er-doped fiber laser based on mechanical exfoliated graphene saturbale absorber,” Chin. J. Lasers 42(8), 0802013 (2015). [CrossRef]
23. H. Chen, S. P. Chen, Z. F. Jiang, and J. Hou, “Versatile long cavity widely tunable pulsed Yb-doped fiber laser with up to 27655th harmonic mode locking order,” Opt. Express 23(2), 1308–1318 (2015). [CrossRef] [PubMed]
24. J. Lee, J. Koo, Y. M. Jhon, and J. H. Lee, “Femtosecond harmonic mode-locking of a fiber laser based on a bulk-structured Bi2Te3 topological insulator,” Opt. Express 23(5), 6359–6369 (2015). [CrossRef] [PubMed]
25. M. Pawliszewska, Y. Ge, Z. Li, H. Zhang, and J. Sotor, “Fundamental and harmonic mode-locking at 2.1 μm with black phosphorus saturable absorber,” Opt. Express 25(15), 16916–16921 (2017). [CrossRef] [PubMed]
26. X. Wang, P. Zhou, X. Wang, H. Xiao, and Z. Liu, “Pulse bundles and passive harmonic mode-locked pulses in Tm-doped fiber laser based on nonlinear polarization rotation,” Opt. Express 22(5), 6147–6153 (2014). [CrossRef] [PubMed]
27. Q. Jia, T. Wang, W. Ma, P. Liu, P. Zhang, B. Bo, and Y. Zhang, “Passively harmonic mode-locked pulses in thulium-doped fiber laser based on nonlinear polarization rotation,” Opt. Eng. 55(10), 106121 (2016). [CrossRef]
28. Y. Zhang, L. P. Hu, T. J. Zhu, J. Xie, and X. B. Zhao, “High yield Bi2Te3 single crystal nanosheets with uniform morphology via a solvothermal synthesis,” Cryst. Growth Des. 13(2), 645–651 (2013). [CrossRef]
29. M. Pang, W. He, P. St J Russell, and J. Russell, “Gigahertz-repetition-rate Tm-doped fiber laser passively mode-locked by optoacoustic effects in nanobore photonic crystal fiber,” Opt. Lett. 41(19), 4601–4604 (2016). [CrossRef] [PubMed]
